Optical fiber sensors are well known and their application fields cover a broad area ranging from physical parameter measurement to chemical and biochemical parameter measurement.
Patents and scientific papers have also been published in the field of chemical and biochemical measurement through luminescent optical fiber sensors. These cover the biomedical field through the measurement of physiological parameters such as pH, O2, glucose, and CO2 concentration in blood.
Another major area involved by the luminescent detection through optical fiber sensors is the biomedical diagnostic domain through optical biopsy. This area involves the evaluation of biological tissues through the measurement of a tissue's auto-fluorescence or through induced fluorescence by specific markers revealing the presence or absence of pathological tissues. These techniques are currently under development but some have reached the clinical level.
Of particular interest is the measurement of temperature through luminescent optical fiber sensors since optical fibers, unlike thermistors and thermocouples, are not affected by microwaves used in thermal treatment of cancers.
Luminescent optical fiber sensors usually work as follows: an excitation wavelength is directed into the optical fiber entrance with appropriate optical components. The excitation light travels through the fiber up to the other end of the fiber, where a luminescent material has been packaged at the fiber tip. The incoming light excites the luminescent material which in turn emits its luminescent light. The material is chosen such that its luminescent light properties (intensity, spectral content, lifetime decay) vary with the parameter to be measured. The luminescent light follows the optical fiber path down to the fiber entrance and is then collected and filtered against the excitation wavelength with proper optics and electronics. Finally, the luminescent properties of the collected light are analysed to deduce the parameter value to be measured.
Most or all of these luminescent optical fiber sensors are packaged at one end of the fiber. Thus, few or none allow distributed measurements, either by spatially distributing the measurement of one parameter or through simultaneous measurement of many parameters, through only one fiber. Furthermore, in some cases, the fact that the sensor is placed at the end of the fiber renders its use less attractive.
For example, it is known that the temperature measurement of intra-arterial walls can be used as a diagnostic tool to detect active arteriosclerotic plaque at risk of disrupting. These active plaques have a temperature which is higher (from 0.1 to 1.5° C.) than normal arterial walls, and the temperature measurement of intra-arterial walls can then be used to detect these plaques. If one measures the temperature of intra-arterial walls with a luminescent optical fiber temperature sensor placed at the end of the fiber, one will use the small and potentially piercing sensing end of the fiber to make contact with the arterial wall. This is a serious disadvantage, since one can accidentally pierce the artery or worse, the active arterial plaque can be broken, which can result in a cardiac stroke.
The same configuration, i.e. the use of the sensor at one end of the fiber, could be used to measure the fluorescence coming from the arterial wall. In this case, the optical fiber is used as a light pipe to make the excitation light reach the arterial wall and to gather part of the luminescent light from the wall and guide it down to the fiber entrance. The luminescent light can then be analysed to identify the type of biological tissue and eventually diagnose the presence of plaques at risk of disrupting. However, to excite and collect the maximum of light level, one needs to put the fiber end in contact with the arterial wall, which can lead to the problems described above. This is also true for any optical fiber extrinsic spectroscopic sensor which collect light (luminescent or not) from biological tissue or from an optical sensing material making contact with that tissue.
Thus the use of conventional optical fiber sensor packaged at one end of the fiber should be prohibited in cases where biological tissue damage can cause health problems.
Therefore, there is a need for a sensor better adapted for a safe in vivo spectroscopy. Moreover, it would be desirable to provide a sensor offering more precise measurement.